Mal will be 51. I could go one of two ways with him. Either he lets go and just becomes the prison drunk, or he mellows out. I'd love for him to settle down with something. Now that he realizes he'll never have Serenity or Inara, maybe he could start looking around at the other women? If there are any women willing to be his partner? He'll definitely have gray hair, but other than that, physicality depends on whether he becomes the drunk or he settles down.
Martha will be 44. She ages gracefully. She's managed to get her scars removed. Like Mal, I'd love to play her settled with someone, but if no one is up for that, I would also be willing for her to be a character who somehow was never able to get in a relationship here. Again, it's up in the air. IF you want your man to wake up next to Martha in the morning hit me up. If not, that's cool.
Sarah will be 35. Oh dear. I need to talk with people about this before I really come up with something. Suffice it to say, I think she'll have matured a bit by then.